Condensed milk cookies

Preheat the oven to 180C/356F/gas mark 4 and line four large baking sheets with baking paper. You may need more or less baking sheets depending on the size of your sheets and oven.
In a stand mixer, cream together the sugar, eggs and vanilla until light and fluffy.
Add the condensed milk and mix until well incorporated.
In a large bowl, mix together the flour, cornflour, cream of tartar, bicarb and salt.
Tip the cold butter cubes in with the flour. Rub the butter into the flour with your hands until no chunks of butter remain. This should take about 5 minutes.
Add the condensed milk mixture to the dry ingredients. Using your hands, combine until you have a uniform dough.
Take care to ensure you have no dry ingredients left on the bottom of the bowl and that all dry flour has been worked into the dough.
You can now either roll the dough into balls and flatten with a fork, or roll the dough out to cut shapes with a cookie cutter, or use a cookie press.
Bake for 12 minutes per tray or until the cookies have a very light golden hue.
Baking time may vary on the thickness and size of the cookies, so keep an eye for visual clues that they are done.
Leave the cookies to cool on the tray for about 5 minutes before transferring them to a cooling rack.

In a stand mixer, or using an electric hand mixer, whisk together the condensed milk, very soft butter and vanilla extract.
Add the icing sugar and continue beating until light and fluffy, about 2-3 minutes.
Sandwich the cookies together with the filling and leave to set for a few hours. They are usually at their best the following day.